Elastic scattering of light has long been used for analyzing random media. Reflectance spectroscopy and imaging are widely used noninvasive methods for measuring the optical properties of random media (e.g., atmosphere, oceans and tissues), including an absorption coefficient (μa) and a reduced scattering coefficient (μs′). These parameters can provide valuable information about the microstructures and the biochemical components of the media, and have been applied in the fields of cloud remote sensing, monitoring of cell apoptosis, skin characterization, cancer detection and the like. Since radiative transfer (RT) describes the propagation of light in random media, the reflectance of scattered light is essentially a difficult problem. Particularly in the case of a short light source-detector distance, the diffusion approximation usually adopted for the RT cannot work. Therefore, it is still difficult to quantify a phase function of a medium from reflectance measurement, particularly to acquire a medium phase function containing basic information of a microenvironment of the relevant medium. In the case of a random light source-detector distance, an accurate analysis model for reflectance is highly desirable. This model will be applied to rapid quantitative evaluation on optical properties, especially on a phase function of a random medium. Those reflectance empirical models in the case of a short light source-detector distance proposed recently have respective limitations. Since the phase function of the scattering medium has a significant influence on the sub-diffuse reflection in the case of a short light source-detector distance, an explicit analysis model that is related to the relation between the sub-diffuse reflectance and the phase function and can deduce the optical parameters (including the phase function) of the random medium from the reflectance distribution is extremely desirable.